1. Field of the Invention
The present invention relates to an improved process for producing a thin glass film by a sol-gel method.
2. Description of the Related Art
A sol-gel method comprises hydrolyzing and poly-condensating a metal alkoxide in a solution containing the metal alkoxide, water and an alcohol (hereinafter referred to as "hydrolyzable solution") to form a porous solid material and heating the porous material to produce a glass material. Since the sol-gel method produces the glass through the hydrolyzable solution, it is easy to uniformly coat a wide substrate with the glass. Then, the sol-gel method is a promising method for coating the substrate with a glass film having a new function of mechanical properties, chemical protection and optical properties.
As described in, for example, "YOGYO KYOKAI-SHI" (Ceramic Society Journal), 90(6), 328-332 (1952), a general procedure of the sol-gel method comprises mixing ethyl silicate, titanium isopropoxide, ethanol, water and HCl as a catalyst to prepare a hydrolyzable solution (a coating liquid), dipping a substrate in the hydrolyzable solution, lifting up the substrate at a constant rate to form a film of the hydrolyzable solution on the substrate and drying and heating the film to obtain a glass film.
Hitherto, the sol-gel method can produce a transparent uniform coating film having a thickness of 0.1 to 0.3 .mu.m. However, though it is tried to produce a glass film having a thickness larger than 0.3 .mu.m, the glass film tends to have various defects such as cracks, white turbidity, and peeling from the substrate. To increase the film thickness, it is known to add glycerin or ethyl ether as a thickener. By the addition of the thickner, the maximum film thickness reaches about 0.5 .mu.m.